IEnumerator(Of T) Interface
Supports a simple iteration over a generic collection.
Assembly: mscorlib (in mscorlib.dll)
'Declaration Public Interface IEnumerator(Of Out T) _ Inherits IDisposable, IEnumerator
- Out T
The type of objects to enumerate.
This type parameter is covariant. That is, you can use either the type you specified or any type that is more derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.
Thetype exposes the following members.
|Dispose||Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources. (Inherited from IDisposable.)|
|MoveNext||Advances the enumerator to the next element of the collection. (Inherited from IEnumerator.)|
|Reset||Sets the enumerator to its initial position, which is before the first element in the collection. (Inherited from IEnumerator.)|
is the base interface for all generic enumerators.
The foreach statement of the C# language (for each in C++, For Each in Visual Basic) hides the complexity of the enumerators. Therefore, using foreach is recommended, instead of directly manipulating the enumerator.
Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection.
Initially, the enumerator is positioned before the first element in the collection. At this position, Current is undefined. Therefore, you must call MoveNext to advance the enumerator to the first element of the collection before reading the value of Current.
If MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. When the enumerator is at this position, subsequent calls to MoveNext also return false. If the last call to MoveNext returned false, Current is undefined. You cannot set Current to the first element of the collection again; you must create a new enumerator instance instead.
The Reset method is provided for COM interoperability. It does not necessarily need to be implemented; instead, the implementer can simply throw a NotSupportedException. However, if you choose to do this, you should make sure no callers are relying on the Reset functionality.
An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and the next call to MoveNext or Reset throws an InvalidOperationException.
The enumerator does not have exclusive access to the collection; therefore, enumerating through a collection is intrinsically not a thread-safe procedure. To guarantee thread safety during enumeration, you can lock the collection during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.
Default implementations of collections in the System.Collections.Generic namespace are not synchronized.Notes to Implementers
Implementing this interface requires implementing the nongeneric IEnumerator interface. The MoveNext and Reset methods do not depend on T, and appear only on the nongeneric interface. The Current property appears on both interfaces, and has different return types. Implement the nongeneric IEnumerator.Current property as an explicit interface implementation. This allows any consumer of the nongeneric interface to consume the generic interface.
In addition, implements IDisposable, which requires you to implement the Dispose method. This enables you to close database connections or release file handles or similar operations when using other resources. If there are no additional resources to dispose of, provide an empty Dispose implementation.
The following example shows an implementation of the interface for a collection class of custom objects. The custom object is an instance of the type Box, and the collection class is BoxCollection. This code example is part of a larger example provided for the ICollection(Of T) interface.
' Defines the enumerator for the Boxes collection. ' (Some prefer this class nested in the collection class.) Public Class BoxEnumerator Implements IEnumerator(Of Box) Private _collection As BoxCollection Private curIndex As Integer Private curBox As Box Public Sub New(ByVal collection As BoxCollection) MyBase.New() _collection = collection curIndex = -1 curBox = Nothing End Sub Private Property Box As Box Public Function MoveNext() As Boolean _ Implements IEnumerator(Of Box).MoveNext curIndex = curIndex + 1 If curIndex = _collection.Count Then ' Avoids going beyond the end of the collection. Return False Else 'Set current box to next item in collection. curBox = _collection(curIndex) End If Return True End Function Public Sub Reset() _ Implements IEnumerator(Of Box).Reset curIndex = -1 End Sub Public Sub Dispose() _ Implements IEnumerator(Of Box).Dispose End Sub Public ReadOnly Property Current() As Box _ Implements IEnumerator(Of Box).Current Get If curBox Is Nothing Then Throw New InvalidOperationException() End If Return curBox End Get End Property Private ReadOnly Property Current1() As Object _ Implements IEnumerator.Current Get Return Me.Current End Get End Property End Class ' Defines two boxes as equal if they have the same dimensions. Public Class BoxSameDimensions Inherits EqualityComparer(Of Box) Public Overrides Function Equals(ByVal b1 As Box, ByVal b2 As Box) As Boolean If b1.Height = b2.Height And b1.Length = b2.Length And b1.Width = b2.Width Then Return True Else Return False End If End Function Public Overrides Function GetHashCode(ByVal bx As Box) As Integer Dim hCode As Integer = bx.Height ^ bx.Length ^ bx.Width Return hCode.GetHashCode() End Function End Class